What is the difference between a Turtle, a Tortoise, and a Terrapin?

The main difference is based on physiology. Tortoises are terrestrial (land dwelling) and possess thick legs and toes and require water for drinking only. There are no tortoises indigenous to Australia.

A freshwater turtle is aquatic and is not capable of swallowing food or mating unless submerged in water. This turtle specie possess webbed feet or paddle-shaped, flipper-like limbs (as in the case of the Pig-nosed or Pitted-shelled turtle) and will only leave the water to lay eggs, bask in the sun or seek more favorable conditions in circumstances such as food shortage or drought. Freshwater turtle kept on dry land will dehydrate, starve, and die slowly and painfully.

"Terrapin" is merely a synonym for "Turtle" and was derived from the North American Indian word "Terrapene".